The function of a high potential iron-sulfur protein, the Rieske iron-sulfur center, in electron transfer and energy transduction of biomembranes is under investigation. Evidence has been obtained for an interaction between this electron carrier and quinone analogues which are inhibitors of electron transport processes. One analogue, UHDBT, inhibits near the site of the Rieske center while a second analogue, DBMIB, appears to interact closely with the iron-sulfur center of the Rieske protein as evidenced by a marked alteration in the spin resonance signal of the center. Further studies of the nature of this interaction will be undertaken. Redox characteristics of the altered iron-sulfur center will be studied in detail and attempts will be made to study this interaction with the purified protein as well. The possible role of the Rieske center as a quinone-binding protein in vivo will be considered in relation to function in electron transport processes.